Staphylococcus aureus is an opportunistic bacterial pathogen involved in severe infections in humans. Most virulence determinants in S. aureus are carried on mobile genetic elements, such as plasmids, bacteriophages and genomic islands. Among these are the S. aureus pathogenicity islands (SaPIs), which carry genes encoding superantigen toxins and other virulence factors. SaPIs are normally stably integrated into the host genome, but become mobilized by specific helper phages, resulting in packaging of the SaPI genomes into transducing particles made from helper-encoded structural proteins. Many SaPIs redirect the assembly pathway of their helpers to form capsids that are smaller than those normally made by the phage. We recently described a new class of SaPIs (including SaPIbov5) that are mobilized by prolate cos phages, which package units of DNA delimited by cos sites, rather than headfuls of DNA. The mechanism of capsid size redirection is completely different from the SaPIs that are mobilized by headful packaging phages, and depends on a SaPI-encoded capsid protein (CP) homolog called Ccm, but the process is not understood. In this developmental project we will investigate this process by a combination of staphylococcal genetics, biochemistry and high resolution cryo-EM. There are three aims: (1) Determine whether Ccm is incorporated into small phage capsids; (2) Determine structures of ?12 (large) and SaPIbov5 (small) capsids; (3) Define the roles of CP and Ccm during capsid assembly. Upon completion of these aims, we will have gained new insights into the process of assembly in prolate phages, mobilization of cos SaPIs, SaPI evolution and the role of SaPIs in the emergence of staphylococcal virulence.